Optical signal quality monitoring apparatus

ABSTRACT

The present invention relates to the quality monitoring of optical signals, which have different symbol rates and generated by different modulation schemes, used for example in a wavelength division multiplexing network. The apparatus according to the invention includes an optical splitter for outputting the input optical signal to a first optical route and a second optical route, an optical coupler for coupling a optical signal from the first optical route with a optical signal from the second optical route, a delay unit provided on the first optical route, and a phase shift unit provided on the first optical route or the second optical route.

PRIORITY CLAIM

This application claims priority from Japanese patent application No.2007-318364 filed on Dec. 10, 2007, which is incorporated herein byreference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an apparatus, which monitors thequality of an optical signal.

2. Description of the Related Art

As optical modulation, phase shift keying (PSK) or differential PSK(DPSK) has attracted attention, because PSK and DPSK modulation has anadvantage that it is less subject to the dispersion of the opticalfiber. Further demodulation is easy in case of DPSK. Multilevelmodulation is also available, and quadrature PSK (QPSK) and differentialQPSK (DQPSK) have been proposed in international patent publicationWO02/51041 A2.

Meanwhile, Akihito Otani et al., “Demonstration of Far-end 160-Gb/sWaveform Measurement after 508-km Transmission in Field Trial withoutTraditional Clock Recovery”, ECOC 2006 Proceedings, Vol. 3, pp. 349-350discloses a configuration which recovers a clock signal regardless of asymbol rate.

SUMMARY OF THE INVENTION

In a network system using a wavelength division multiplexing (WDM)technique, optical signals generated by various modulation schemes andhaving various bit rates or symbol rates are used simultaneously.Therefore, a quality monitoring apparatus needs to handle variousoptical signals.

Therefore, an object of the present invention is to provide an opticalsignal quality monitoring apparatus, which can handle a plurality ofoptical signals having different symbol rates and modulation formats.

According to the present invention, an optical signal quality monitoringapparatus has an optical splitter, an optical coupler, a delay unit anda phase shift unit.

The optical splitter divides an input optical signal to a first opticalsignal and a second optical signal, outputs the first optical signal toa first optical route, and outputs the second optical signal to a secondoptical route. The optical coupler couples the first optical signal withthe second optical signal, and outputs a coupled optical signal to ameasurement unit which measures the quality of the optical signal. Thedelay unit is provided on the first optical route, and the phase shiftunit is provided on the first optical route or the second optical route.

Preferably, the delay unit introduces a delay into the optical signalpassing through it such that the optical signal from the first opticalroute is delayed for the integral multiple of a symbol period of theinput optical signal compared to the optical signal from the secondoptical route at the optical coupler. The phase shift unit introduces aphase shift into the optical signal passing through it, and the amountof phase shift is varied.

According to another aspect of the invention, an optical signal qualitymonitoring apparatus has a first optical splitter, a first opticalcoupler, a delay unit, a second optical splitter, a second opticalcoupler, a first phase shift unit and a second phase shift unit.

The first optical splitter divides an input optical signal to a firstand a second optical signal, outputs the first optical signal to a firstoptical route, and outputs the second optical signal to a second opticalroute. The first optical coupler couples the first optical signal withthe second optical signal, and outputs a coupled optical signal to ameasurement unit, which measures the quality of the optical signal. Thedelay unit is provided on the first optical route. The second opticalsplitter is provided on the first optical route or the second opticalroute, and divides an input optical signal to a third optical signal anda fourth optical signal, outputs the third optical signal to a thirdoptical route, and outputs the fourth optical signal to a fourth opticalroute. The second optical coupler couples the third optical signal withthe fourth optical signal, and outputs a coupled optical signal. Thefirst phase shift unit is provided on the third optical route, and thesecond phase shift unit is provided on the fourth optical route.

Due to the delay unit, the optical signal from the first optical routeis delayed for the integral multiple of the symbol period of the inputoptical signal compared to the optical signal from the second opticalroute at the first optical coupler. Favorably, an amount of phase shiftintroduced by the first phase shift unit into the optical signal passingthrough it is varied, and a amount of phase shift introduced by thesecond phase shift unit into the optical signal passing through it iseither the same or larger by π, i.e. reversed, compared to the amount ofphase shift introduced by the first phase shift unit.

Advantageously, the second phase shift unit introduces the phase shift,which is in phase as the first phase shift unit does, in case the inputoptical signal is DPSK or DQPSK signal. On the contrary, the secondphase shift unit introduces the phase shift, which is reversed phase asthe first phase shift unit does, in case the input optical signal isamplitude shift keying (ASK) modulation signal.

Further objects and advantages of the present invention will be apparentfrom the following description of the preferred embodiments of theinvention as illustrated in the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of an optical signal quality monitoringapparatus according to the invention;

FIG. 2 shows periodically varying phase shift introduced by a phaseshift unit;

FIG. 3 is an explanation drawing of the quality monitoring for anoptical DPSK modulation signal according to the invention;

FIGS. 4 a and 4 b are explanation drawings of the quality monitoring foran optical DQPSK modulation signal according to the invention; and

FIG. 5 is a block diagram of an optical signal quality monitoringapparatus according to another embodiment of the invention.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 is a block diagram of an optical signal quality monitoringapparatus according to the invention. The apparatus includes an opticalsplitter 1, a delay unit 2, a phase shift unit 3, an optical coupler 4and a measurement unit 5.

The optical splitter 1 divides an input optical signal into a firstoptical signal and a second optical signal, outputs the first opticalsignal to an optical route 11, and outputs the second optical signal toan optical route 12. The optical coupler 4 couples the first opticalsignal passing through the optical route 11 with the second opticalsignal passing through the optical route 12, and the measurement unit 5measures the quality of the input optical signal based on an opticalsignal from the optical coupler 4.

The delay unit 2 delays the first optical signal such that the firstoptical signal is delayed for the integral multiple of the symbol periodof the input optical signal compared to the second optical signal at thecoupler 4. In other word, the amount of delay of the first opticalsignal with reference to the second optical signal at the coupler 4 isn*(1/S), where n is an integer and S is a symbol rate of the inputoptical signal. The phase shift unit 3 shifts the phase of the firstoptical signal. In the embodiment, the delay unit 2 and the phase shiftunit 3 are provided on the same optical route. However, the delay unit 2and the phase shift unit 3 can be provided on different optical routes.Further, in the embodiment, the phase shift unit 3 is placed downstreamof the delay unit 2. However, the phase shift unit 3 can be placedupstream of the delay unit 2.

For the normal demodulation process, the amount of delay at the coupler4 is one symbol period. However, the purpose of the invention is not todemodulate the optical signal, but to determine the quality of theoptical signal by monitoring the waveform. Thus, the amount of delay canbe the integral multiple of the symbol period. Therefore, it is possibleto monitor a plurality of optical signals having different symbol rateswithout changing the amount of delay introduced at the delay unit 2 bysetting the amount of delay equal to the least common multiple of symbolrates of optical signals to be monitored to the delay unit 2.

FIG. 2 shows periodically varying phase shift added or introduced by thephase shift unit 3. The phase shift unit 3 is for example a heater,which changes refractive index of the optical fiber by adding a heat,and introduces a phase shift, the amount of which is periodically variedin the range of at least 0 to π, into the optical signal passing throughit. In the embodiment shown in FIG. 2, the amount of phase shift ischanged between 0 and 2π periodically. One period of the phase shift,i.e. the time from 0 shift to 2π shift in the embodiment, should belarge enough compared to the symbol period of the input optical signalto be monitored. For example, for the optical signal having symbol ratemore than 10 Gbaud, one period of the phase shift is set to more than 1second. In the embodiment, the change of phase shift is ramp-shaped.However, the invention is not limited to this, and it is possible to useother shape like sinusoidal-shaped or triangle-shaped. Further,nonperiodic phase variation can be used.

FIG. 3 is an explanation drawing of the quality monitoring for anoptical DPSK modulation signal according to the invention. In FIG. 3,the optical signal shows optical phases of the optical DPSK signalcorresponding to data sequence indicated at the first line. At theoptical coupler 4, the optical signal is coupled with another opticalsignal, which is delayed for several bits at the delay unit 2. However,the amount of delay introduced by the delay unit 2 is not accurate, andthere is some phase error at the optical coupler 4. In FIG. 3, thedelayed optical signal shows the optical phases of a 1 bit delayedoptical DPSK signal at the optical coupler 4, and it has the offset by adue to above-described reason.

In a normal demodulator, the phase offset or error is compensated usinga complex and expensive feedback loop. However, the apparatus accordingto the invention uses the phase shift unit 3, which shifts optical phaseas shown in FIG. 2, instead of the complex feedback loop. Because of thephase shift introduced by the phase shift unit 3, the phase offset a atthe optical coupler 4 varies 0 to 2π. FIG. 3 also shows waveforms outputfrom the optical coupler 4, in case the offset α equal to 0, π/4, π/2and 3π/4, respectively. In case the offset α equals to 0, the opticalcoupler 4 outputs an optical ASK signal, which is used in the normaldemodulation process. With increasing the offset α from 0, the maximumamplitude of the ASK signal decreases, and the minimum amplitude of theASK signal increases, as shown in case of α=π/4. When the offset αbecomes π/2, the maximum amplitude is the same as the minimum amplitudeas shown in FIG. 3. With increasing the offset α from π/2, the opticalcoupler 4 outputs the ASK signal, the maximum and the minimum amplitudeof which are inverted compared to the signal while the offset α is inthe range of 0 to π/2, as shown in case of α=3π/4.

The measurement unit 5 receives the optical signal output from theoptical coupler 4, and determines the maximum eye opening points of thesignal. Here, the maximum eye opening points means signal points thatthe difference between the maximum amplitude and the minimum amplitudebecomes the maximum. More specifically, the maximum eye opening pointsare instants when the offset α is 0 or π according to the embodiment.The measurement unit 5 uses each predetermined period of the signal fromthe coupler 4 including the determined maximum eye opening point formeasuring Q factor of the input optical signal. More specifically, themeasurement unit 5 extracts a clock signal and a trigger from theoptically sampled optical signal, for example, using the methoddescribed in Akihito Otani et al., “Demonstration for Far-end 160-Gb/sWaveform Measurement after 508-km Transmission in Field Trial withoutTraditional Clock Recovery”. Further, the measurement unit 5 convertsthe optical signal to an electrical signal, and samples the electricalsignal. Then, the measurement unit 5 determines maximum eye openingpoints of the sampled electrical signal, determines parts of the sampledelectrical signal used for Q factor measurement based on the maximum eyeopening points, and measures Q factor from a distribution of the maximumamplitude and minimum amplitude in the determined parts of the signal.For example, the period of the signal for Q factor measurement isdecided by the phase shift period of the phase shift unit 3. Further,the period of the signal for Q factor measurement can be decided basedon the ratio of eye opening to the maximum eye opening. In other word, Qfactor measurement is performed while difference between the maximum andthe minimum amplitude is more than the value, which is decided based onthe maximum eye opening.

FIGS. 4 a and 4 b are explanation drawings of the quality monitoring foran optical DQPSK modulation signal according to the invention. In thedemodulation process of the optical DQPSK signal, the optical DQPSKsignal is divided to a first DQPSK signal and a second DQPSK signal.Then, the first DQPSK signal is delayed for 1 bit, and phase-shifted by+π/4 or −π/4. Finally, the second DQPSK signal is coupled with thedelayed and phase-shifted first DQPSK signal at a coupler to output anoptical ASK signal.

For example, in case the optical DQPSK signal uses optical phases of 0,π/2, π and 3π/2, optical phases of the first DQPSK signal is π/4, 3π/4,5π/4 and 7π/4 at the coupler as indicated by coordinates 51, 53, 54 and52 in FIG. 4 a, respectively. The coupler combines the first DQPSKsignal having an optical phase of π/4, 3π/4, 5π/4 or 7π/4 with thesecond DQPSK signal having an optical phase of 0, π/2, π or 3π/2. InFIG. 4 a, only an optical phase 0 is indicated by a reference numeral 50for the second DQPSK signal. As apparent from FIG. 4 a, amplitude of thecoupled signal output from the coupler has 2 levels. In case the opticalphase of the second DQPSK signal is 0 as shown in FIG. 4 a, the maximumlevel is output when an optical phase of the first DQPSK signal is π/4or 7π/4, and the minimum level is output when an optical phase of thefirst DQPSK signal is 3π/4 or 5π/4. However, as already explained, thereis some phase offset at the coupler. FIG. 4 b shows optical phases ofthe first DQPSK signal at the coupler, in case there is a phase error oroffset β. In this case, as apparent from FIG. 4 b, the coupler outputs asignal, amplitude of which has 4 levels. That is the eye opening becomesmaller.

In the apparatus, an input optical DQPSK signal is divided into a firstDQPSK signal and a second DQPSK signal at the optical splitter 1. Thefirst DQPSK signal passes through the optical route 11, and the secondDQPSK signal passes through the optical route 12. One or more bits delayand the phase shift shown in FIG. 2 are introduced into the first DQPSKsignal by the delay unit 2 and the phase shift unit 3. Thus, the opticalcoupler 4 outputs the ASK signal when the phase offset β is 0, π/2, π or3π/2, and outputs the signal having 4 levels in other cases. Themeasurement unit 5 receives the optical signal output from the opticalcoupler 4, determines the maximum eye opening points of the signal,determines parts of the signal used for Q factor measurement based onthe maximum eye opening points, and measures Q factor from adistribution of the maximum amplitude and minimum amplitude in thedetermined parts of the signal. In case of the DQPSK signal, the maximumeye opening points are obtained when the signal becomes 2 levels.

As described above, the apparatus according to the invention can monitorboth optical DPSK and DQPSK signals. Since the complex feedback loop isnot required for the apparatus, it is possible to reduce the cost of theapparatus. Further, the apparatus can monitor a plurality of opticalsignals having different symbol rates by setting the integral multipleof the symbol period for the amount of delay to the delay unit 2.

FIG. 5 is a block diagram of an optical signal quality monitoringapparatus according to another embodiment of the invention. Theapparatus according to the embodiment can monitor an optical ASK signalin addition to the optical DPSK and DPQSK signals. In FIGS. 1 and 5, thesame reference numeral is used for the element having the same orsimilar function, and hereinafter the explanation is omitted for theelement having the same reference numeral in FIG. 1. In the embodiment,an optical splitter 6 further divides the first optical signal to athird and a fourth optical signal, outputs the third optical signal toan optical route 111, and outputs the fourth optical signal to anoptical route 112. A phase shifts unit 31 shifts the optical phase ofthe third optical signal, and a phase shift unit 32 shifts the opticalphase of the fourth optical signal. An optical coupler 7 couples thethird optical signal with the fourth optical signal, and outputs thecoupled signal to the optical coupler 4 via the optical route 11.

In the embodiment, the phase shift unit 31 shifts the phase of theoptical signal passing through it. The amount of phase shift introducedby the phase shift unit 31 is varied at least in the range of 0 to π. Inthe embodiment shown in FIG. 2, the amount of phase shift is changedbetween 0 and π. The phase shift unit 32 also introduces the phase shiftinto the optical signal passing through it. The phase shift unit 32 isadapted to introduce the same phase shift or reverse phase shift withreference to the phase shift introduced by the phase shift unit 31.

More specifically, in case of monitoring the optical DPSK or DQPSKsignal, the amount of phase shift introduced by phase shift units 31 and32 are the same. On the contrary, in case of monitoring the optical ASKsignal, the amount of phase shift introduced by the phase shift units 32is reversed compared to the amount of phase shift introduce by the phaseshift unit 31. In other words, when the phase shift unit 31 shifts thephase by θ, the phase shift unit 32 shifts the phase by θ+π.

In case the phase shift units 31 and 32 introduce the same phase shift,the optical coupler 7 outputs the same signal, but phase shifted, as theinput to the optical splitter 6. Therefore, an arrangement having theoptical splitter 6, the phase shift unit 31, the phase shift unit 32 andthe optical coupler 7 is equivalent to the phase shift unit 3 in FIG. 1,and works as the same way as already explained using FIG. 1. In case thephase shift units 31 and 32 introduce reverse phase shift, the opticalcoupler 7 does not output a signal. Therefore, the optical coupler 4just outputs the optical signal from the optical route 12, i.e. theinput optical ASK signal. Therefore, in either case, the optical coupler4 outputs the optical ASK signal to the measurement unit 5.

In the embodiment, the amount of phase shift introduced by the phaseshift unit 32 is switchable between in phase and reverse phase withreference to the one introduced by the phase shift unit 31, and controlunit, not shown in figures, selects in phase or reverse phase based onthe modulation format applied to the input optical signal. With thisconfiguration, it is possible to monitor the optical ASK signal inaddition to the optical DPSK and DQPSK signals.

In FIG. 5, the arrangement having the optical splitter 6, the phaseshift unit 31, the phase shift unit 32 and the optical coupler 7 isprovided on the optical route 11. However the arrangement can beprovided on the optical route 12. Further the arrangement can beprovided upstream of the delay unit 2.

Many modifications and variations will be apparent those of ordinaryskilled in the art. The embodiments was chosen and described in order tobest explain the principles of the invention. It should be understoodthat the present invention is not limited to the specific embodimentsdescribed in the specification, except as defined in the appendedclaims.

For example, the phase shift range in the embodiment described above isat least 0 to π. However, it is possible to use π to 2π. In other words,it is possible to apply any range if the difference between the maximumphase shift and the minimum phase shift introduced by the phase shiftunit is more than or equal to π. Further, in case only optical DQPSK andASK signals are monitored, the phase shift range in the phase shift unitcan be, for example, 0 to π/2. In this way, the required phase shiftrange depends on modulation formats to be monitored, and the person inthe art can select the range based on signals to be monitored.

1. An optical signal quality monitoring apparatus comprising: an opticalsplitter for dividing an input optical signal and outputting to a firstoptical route and a second optical route; an optical coupler forcoupling a optical signal from the first optical route with a opticalsignal from the second optical route; a delay unit provided on the firstoptical route; and a phase shift unit provided on the first opticalroute or the second optical route, wherein the phase shift unitintroduces a periodic dynamic phase shift as a function of time into theoptical signal passing through the optical signal quality monitoringapparatus.
 2. The optical signal quality monitoring apparatus accordingto claim 1, wherein the delay unit introduces a delay into the opticalsignal passing through it such that the optical signal from the firstoptical route is delayed for an integral multiple of a symbol period ofthe input optical signal compared to the optical signal from the secondoptical route at the optical coupler, and the phase shift unitintroduces a varying phase shift into the optical signal passing throughit.
 3. The optical signal quality monitoring apparatus according toclaim 2, wherein the difference between the minimum phase shift and themaximum phase shift introduced by the phase shift unit is more than orequal to π.
 4. The optical signal quality monitoring apparatus accordingto claim 1, further comprising a measurement unit for receiving anoptical signal output from the optical coupler, wherein the measurementunit uses signal parts including the maximum eye opening point of thereceived signal for determining the quality of the input optical signal.5. An optical signal quality monitoring apparatus comprising: a firstoptical splitter for dividing an input optical signal and outputting toa first optical route and a second optical route; a first opticalcoupler for coupling a optical signal from the first optical route witha optical signal from the second optical route; a delay unit provided onthe first optical route; and an arrangement provided on the firstoptical route or the second optical route, wherein the arrangementcomprises: a second optical splitter for dividing an optical signalinput to it and outputting to a third optical route and a fourth opticalroute; a second optical coupler for coupling a optical signal from thethird optical route with a optical signal from the fourth optical route;a first phase shift unit provided on the third optical route; and asecond phase shift unit provided on the fourth optical route, whereinthe first phase shift unit introduces a periodic dynamic phase shift asa function of time into the optical signal passing through the opticalsignal quality monitoring apparatus, and the second phase shift unitintroduces a periodic dynamic phase shift as a function of time into theoptical signal passing through the optical signal quality monitoringapparatus.
 6. The optical signal quality monitoring apparatus accordingto claim 5, wherein the delay unit introduces a delay into the opticalsignal passing through it such that the optical signal from the firstoptical route is delayed for an integral multiple of a symbol period ofthe input optical signal compared to the optical signal from the secondoptical route at the first optical coupler, the first phase shift unitintroduces a varying phase shift into the optical signal passing throughit, and the second phase shift unit introduces a phase shift into theoptical signal passing through it, wherein the phase shift introduced bythe second phase shift unit is in phase or reverse phase with referenceto one introduced by the first phase shift unit.
 7. The optical signalquality monitoring apparatus according to claim 6, wherein the phaseshift introduced by the second phase shift unit is in phase when theinput optical signal is a differential phase shift keying or adifferential quadrature phase shift keying modulation signal, and thephase shift introduced by the second phase shift unit is reverse phasewhen the input optical signal is an amplitude shift keying modulationsignal.